Magnetic fields and their evolution drive high energy ultraviolet emission on the sun. The brightest ultraviolet solar regions, plage, occur in regions with strong magnetic fields. A region's magnetic field and corresponding plage evolves as new magnetic fields emerge or old magnetic fields decay. One theory of magnetic field emergence hypothesizes cool, dense plasma emerges with new magnetic fields. The cool, dense plasma, called arch filaments, may be important for a region's evolution, since arch filaments connect opposing magnetic fields. The above IRIS observation shows a plage's long term (8 days) evolution in the core of an ultraviolet Mg II spectral line. Initially, the plage linearly extends east to west (left to right); however, the plage morphs into an inverted U shape over the next few days. The U shaped plage's core contains a few dark S-shaped structures. The dark S-shaped structures are arch filaments, which are dark because they contain relatively cool plasma compared to the surrounding plasma. The arch filaments remain for a few days but eventually dissipate. After the arch filaments dissipate, the plage slowly fades away due to decaying magnetic fields. Long term monitoring of plage, like the observation above, may shed new light on how such regions evolve.